Process For Directed Fracking Of An Underground Formation Into Which At Least One Directional Well Has Been Sunk

ABSTRACT

A process for fracking an underground formation, comprising at least the steps of
     a) sinking a directional well ( 1 ) comprising a quasi-vertical section ( 11 ) and a quasi-horizontal section ( 12 ) into the underground formation,   b) introducing hollow bodies (HB) and a detonatable free-flowing explosive (FE) into the directional well ( 1 ) and   c) initiating the detonation in the directional well ( 1 ),
 
wherein the hollow bodies (HB) have a density (D HB ) and the detonatable free-flowing explosive (FE) has a density (D FE ) and (D HB ) is less than (D FE ), and the end of process step b) is followed by and process step c) is preceded by a rest phase, the result of which is that the hollow bodies (HB) float in the detonatable free-flowing explosive (FE).

The present invention relates to a process for fracking an underground formation into which at least one directional well has been sunk by introducing hollow bodies (HB) and a detonatable, free-flowing explosive (FE) into the directional well and then initiating the detonation in the directional well.

The process according to the invention can be used for development of underground deposits. Suitable underground deposits are, for example, gas and mineral oil deposits, bitumen and heavy oil deposits, coal deposits and ore deposits. In addition, the process according to the invention can be used for underground gasification of coal seams, for underground leaching in metal extraction, for release of rock pressure and for modification of stress fields in geological formations, for water production from underground deposits, and for development of underground geothermal deposits.

In the development of the aforementioned deposits, it is common practice to hydraulically fracture parts of the underground formation, in order to increase the flow of fluids into and/or out of the underground formation. Hydraulic fracturing (or fissuring of an underground formation), fracking for short, is understood to mean the occurrence of a fracture event in the rock surrounding a well as a result of the hydraulic action of a liquid or gas pressure on the rock of the underground formation.

In the last few years, water-based fracking has become ever more important. This involves using fracking fluids comprising water, gel formers and optionally crosslinkers. The use of crosslinkers leads to spontaneous gel formation within a few minutes. In the case of water-based fracking, a fracking fluid is injected under high pressure through an injection well into the rock stratum to be fractured or fissured.

The fracking fluid is pumped into the stratum to be fractured or fissured at a pressure sufficient to divide or fracture the underground formation. This widens natural fissures and cracks present, which have been formed in the course of evolution of the geological formation and in the event of subsequent tectonic movements, and produces new cracks, crevices and fissures, also called fracs or hydrofracs. Proppants such as sand may be added to the fracking fluid.

The alignment of the hydrofracs thus induced depends particularly on the state of stress existing in the underground formation. The pressure level with which the fracking fluid is pumped into the formation depends on the properties of the rocks and the stress fields in the underground formation. A disadvantage of these processes is that the water-based fracking fluid has to be injected into the underground formation with enormously high pressures, and so these processes are inconvenient and costly. Furthermore, only very limited fissuring of the underground formation is possible by water-based fracking, since the pressure of the fracking fluid injected cannot be increased without limitation. A further disadvantage is that the formation of cracks, crevices and fissures is undirected and depends particularly on the stress state of the underground formation. Directed fracking or directed fissuring of underground rock formations is not possible by these processes.

In order to achieve greater fissuring of underground formations, the prior art also describes the use of explosive. For instance, EP 1 046 879 describes a process for blasting rock masses in which explosive charges with electronic detonators are introduced into a multitude of boreholes and the detonators subsequently detonate the explosive charges in the multitude of boreholes in the rock mass with a specific delay interval. Disadvantages of this process are that a multitude of boreholes have to be introduced into the rock mass, and that the use of a multitude of explosive charges and detonators is necessary. This makes the process inconvenient and costly. Directed fissuring of the rock masses is additionally impossible by this process.

RU 2 333 363 and RU 2 339 818 describe processes for obtaining gas from coal deposits. In these processes, a directional well is sunk from the surface into the gas-bearing stratum of the coal deposit, provided with explosive charges and then blasted. This achieves fissuring of the coal deposit. Natural gas escapes through the cracks and fissures which have formed in the coal deposit, and is subsequently produced. These processes do enable more extensive fissuring, but a disadvantage is that this fissuring is undirected and thus difficult to control. RU 2 333 191 describes an explosive mixture comprising ammonium nitrate, hydrocarbons and cenospheres. The cenospheres have neutral buoyancy in the explosive mixture and are distributed homogeneously, such that the explosion is undirected.

The above-described processes have the disadvantage that the fissuring of the underground formation proceeds in an undirected manner. As a result, it is also possible to destroy rock strata which delimit productive strata, for example an oil-bearing stratum, from adjacent strata, for example formation water-bearing strata. In such a case, the fissuring of the underground formation is actually counter-productive, since the fissuring of the adjoining rock stratum causes waterlogging of the productive stratum and thus disrupts the production process.

Many deposits additionally have a stratified structure in which horizontal productive layers of great thickness are separated from one another by horizontal nonproductive impervious rock strata of lower thickness. Undirected fissuring cannot establish an effective hydrodynamic connection between the productive strata. There was therefore a need for improved processes for fissuring (fracking) of underground formations, which have the disadvantages of the processes described in the prior art only to a reduced degree, if at all.

It is thus an object of the present invention to provide an improved process for fracking of underground formations, with which effective, preferably predominantly vertically directed, fissuring of an underground formation is achieved. The process shall additionally establish an effective hydrodynamic connection between horizontally stratified productive strata separated from one another by nonproductive impervious rock strata.

This object is achieved in accordance with the invention by the following process for fracking an underground formation, comprising at least the steps of:

-   a) sinking a directional well (1) comprising a quasi-vertical     section (11) and a quasi-horizontal section (12) into the     underground formation, -   b) introducing hollow bodies (HB) and a detonatable free-flowing     explosive (FE) into the directional well (1) and -   c) initiating the detonation in the directional well (1),     wherein the hollow bodies (HB) have a density (D_(HB)) and the     detonatable free-flowing explosive (FE) has a density (D_(FE)) and     (D_(HB)) is less than (D_(FE)), and the end of process step b) is     followed by and process step c) is preceded by a rest phase, the     result of which is that the hollow bodies (HB) float in the     detonatable free-flowing explosive (FE).

The process according to the invention enables the directed fracking of an underground formation, preferably predominantly in the vertical direction, in order thus effective establishment or improvement of hydrodynamic connections between horizontal productive strata separated from one another by horizontal impervious rock strata. According to the invention, this directed fissuring is achieved by the guidance of the detonation energy.

“Fracking” is understood in accordance with the invention to mean the controlled induction of a fracture event in the rock surrounding a well by the pressure of detonation. The fracking causes directed fissuring of the rock surrounding the well.

The reference numerals in the present context are defined as follows:

-   1 directional well -   11 quasi-vertical section of the well (1) -   12 quasi-horizontal section of the well (1) -   13 quasi-horizontal section of the well (1) after the detonation -   2 deposit -   2 a productive stratum of the deposit 2 -   3 free-flowing explosive (FE) comprising hollow bodies (HB) -   4 spherical hollow bodies (HB) with positive buoyancy -   5 spherical hollow bodies (HBn) with neutral buoyancy -   6 fissured zone -   7 tamping composition of the free-flowing explosive (3) -   8 feed pipe of the well (1) -   9 fissures in the rock -   10 cylindrical hollow body (HB) -   20 partial coating of the cylindrical hollow body (HB); (10) -   21 partial coating on the inside of the cylindrical hollow body     (HB); (10) -   22 partial coating on the outside of the cylindrical hollow body     (HB); (10) -   23 intermediate strata (nonproductive strata of the deposit 2) -   24 detonator -   25 ore deposit -   26 injection well -   27 production well -   28 alkaline flooding solution -   29 productive solution -   30 coiled tubing -   61 fissured zone in the deposit (injection zone) -   62 fissured zone in the deposit (production zone) -   71 borehole fluid

Process Step a)

In process step a), a directional well (1) is sunk into the underground formation. Techniques for sinking directional wells into underground formations are known to those skilled in the art and are described, for example, in EP 0952300, U.S. Pat. No. 487,008, RU 2451150, EP 1129272.

According to the invention, the directional well (1) has a quasi-vertical section of the well (11) and a quasi-horizontal section of the well (12).

Thus, the present invention also relates to a process in which the directional well (1) has a quasi-vertical section (11) and a quasi-horizontal section (12).

“Vertical” is generally understood to mean an axis (gravity direction) which is aligned to the center of the earth and runs at right angles to the surface of the earth.

“Horizontal” is generally understood to mean a plane (horizontal plane) aligned parallel to the surface of the earth and at right angles to gravity direction.

According to the invention, the quasi-vertical section of the well (11) may deviate by up to 40°, preferably by up to 25° and more preferably by up to 15° from gravity direction.

“Quasi-vertical” is thus understood in accordance with the invention to mean not exclusively sections of a well which exactly follow gravity direction but also sections of a well which deviate by up to a maximum of 40°, preferably by up to a maximum of 25° and more preferably by up to a maximum of 15° from gravity direction.

According to the invention, the quasi-horizontal section of the well (12) may deviate from the horizontal plane by up to a maximum of 30°. The deviation may be positive, in which case the quasi-horizontal section of the well (12) has a positive slope in the direction of the surface of the earth. The deviation from the horizontal plane may also be negative, in which case the quasi-horizontal section of the well (12) has a negative slope in the direction of the center of the earth. “Quasi-horizontal” is thus understood in accordance with the invention to mean sections of a well which deviate by a maximum of +/−30°, preferably by up to a maximum of +/−20° and more preferably by up to a maximum of +/−10° from the horizontal plane.

The quasi-vertical section of the well (11) and the quasi-horizontal section of the well (12) of the well (1) are connected by a curved section.

The length of the quasi-vertical section of the well (11) may vary within wide ranges and depends on the depth of the underground formation which is to be fracked. The length of the quasi-vertical part of the well (11) is generally in the range from 100 to 10 000 meters, preferably in the range from 100 to 4000 meters, more preferably in the range from 100 to 2000 meters and especially in the range of 100 to 1000 meters.

The position of the quasi-horizontal section of the well (12) likewise depends on the position of the underground formation which is to be fracked and may vary within wide ranges. The length of the quasi-horizontal section of the well (12) is generally in the range from 20 to 5000 meters, preferably in the range from 20 to 2000 meters and more preferably in the range from 20 to 1000 meters.

The quasi-vertical section of the well (11) is generally stabilized by customary techniques known to those skilled in the art. This can be accomplished, for example, by cementing or by the introduction of a feed pipe (8). The curved part of the well which connects the quasi-vertical part of the well (11) and the quasi-horizontal part of the well (12) is also typically stabilized by the above-described methods, more particularly by the introduction of a feed pipe (8). The feed pipe (8) is generally made of metal.

The quasi-horizontal section of the well (12) is typically stable at least in the short term after the sinking of the well. In this case, the quasi-horizontal section of the well (12) remains unstabilized, i.e. unlined and uncemented. If the geomechanical studies of the underground formation show that the quasi-horizontal section of the well (12) is unstable even in the short term, this section is lined temporarily, for example with plastic pipes without cementing.

It is also possible to stabilize parts of the quasi-horizontal section of the well (12), for example by the above-described methods, for example cementing or introduction of a feed pipe (8). Typically, however, the only part of the quasi-horizontal section of the well (12) which is stabilized permanently is that in which no detonation is performed for fracking of the surrounding masses of rock.

The process according to the invention is especially suitable for fracking of deposits (2) having a stratified structure. In these deposits, productive strata (2 a) are separated by nonproductive intermediate strata (23). “Productive strata” (2 a) are understood to mean those strata which comprise a raw material which is to be produced. This may comprise natural gas, mineral oil or metals (ores).

The invention thus also relates to a process in which the underground formation comprises a deposit (2) which has a stratified structure and in which a multitude of productive strata (2 a) are separated from one another by a multitude of nonproductive intermediate strata (23).

A “multitude” is understood to mean the number of productive strata (2 a) and nonproductive intermediate strata (23). The number of productive strata (2 a) and nonproductive intermediate strata (23) in the deposit is typically in the range from 3 to 10.

The nonproductive strata (23) may be impervious rock or clay strata. The productive strata (2 a) generally have a thickness of 2 to 20 meters, preferably 2 to 10 meters. The thickness of the nonproductive strata (23) is typically lower and is in the range from a few centimeters to 2 meters, preferably in the range from 0.5 meter to 1 meter.

The quasi-horizontal part of the well (12) is preferably positioned in the lower region of the deposit (2). Preferably into the lowermost productive stratum (2 a) of the deposit (2).

Process Step b)

In the case of use of explosives, both the legal requirements and the workplace safety regulations should be noted. The process according to the invention is used only for civil extraction, i.e. to obtain raw materials. In the process according to the invention, no nuclear explosives are used.

In process step b), hollow bodies (HB) and a detonatable free-flowing explosive (FE) are introduced into the directional well (1). The hollow body (HB) has a density (D_(HB)) less than the density (D_(FE)) of the detonatable free-flowing explosive.

The hollow bodies (HB) used are bodies having one or more hermetic cavities. “Hermetic” in this context means that the cavity is encased by an outer layer which is very substantially impervious to liquids. The cavity is filled with a gas. The gases may be selected as desired, provided that the density (D_(HB)) of the hollow body (HB) is less than the density (D_(FE)) of the detonatable free-flowing explosive (FE). The cavity of the hollow body (HB) is preferably filled with a gas. Useful gases include nitrogen, carbon dioxide, hydrogen, oxygen or air. The cavity of the hollow body (HB) is preferably filled with air.

The present invention thus also relates to a process in which the density (D_(HB)) of the hollow bodies (HB) is in the range from 0.2 to 0.9 g/cm³.

The shape of the hollow body (HB) is uncritical for the performance of the process according to the invention. It is thus possible to use hollow bodies (HB) of any desired geometric shape which meets the aforementioned requirements.

For practical reasons, however, preference is given to using spherical or cylindrical hollow bodies (HB) having one or more cavities. The hollow bodies (HB) having more than one cavity may, for example, be foamed polymers such as expanded polystyrene, or porous ceramic materials.

Preference is given to using hollow bodies (HB) having only one cavity, preference being given here too to spherical or cylindrical hollow bodies (HB).

The spherical hollow bodies (HB) used may, for example, be hollow spheres having an outer shell of a polymer material or a ceramic material or of glass. The spherical hollow bodies (HB) preferably have a density in the range from 0.2 to 0.9 g/cm³. The diameter of the spherical hollow bodies (HB) is generally in the range from 1 μm to 5 mm, preferably in the range from 1 μm to 1000 μm and especially in the range from 1 μm to 300 μm. It is also possible to use spherical hollow bodies (HB) having greater or smaller diameters.

Particularly preferred spherical hollow bodies (HB) are cenospheres having a density in the range from 0.2 to 0.9 g/cm³, more preferably in the range from 0.3 to 0.8 g/cm³, and particle sizes in the range from 1 to 300 μm, preferably in the range from 50 to 200 μm.

Cenospheres are obtained from the fly ash in the combustion of coal or hard coal. Cenospheres are hollow spheres having a shell comprising silicon oxide, aluminum oxide and iron oxide. The cenospheres are separated from the fly ash from the coal or hard coal combustion by methods known to those skilled in the art, for example by means of hydrodynamic methods or gravity methods. The cenospheres are formed in the course of coal combustion as a result of thermochemical conversion of mineral coal components and the crystallization of these components during the cooling process.

The present invention thus also relates to a process in which the hollow bodies (HB) used are cenospheres having a shell comprising silicon oxide, aluminum oxide and iron oxide.

The cylindrical hollow bodies (HB) are preferably pipes or tubes having a hermetic cavity having a density in the range from 0.2 to 0.9 g/cm³. The pipes or tubes are preferably manufactured from a polymer material, for example from polyethylene. The cavity of the cylindrical hollow bodies (HB) is preferably filled with a gas, particular preference being given to air.

The cylindrical hollow bodies (HB) may have a partial coating (20) comprising, for example, half of the arc length of the pipe circumference of the cylindrical hollow body (HB). The partial coating (20) is preferably a metal coating. The partial coating (20) may be on the inside of the cylindrical hollow body (HB) (partial coating (21)) or on the outside of the cylindrical hollow body (HB) (partial coating (22)).

The partial coating (20) of the cylindrical hollow body (HB) facilitates the alignment, described hereinafter, of the cylindrical hollow body (HB) in the quasi-horizontal section of the well (12).

The diameter of the cylindrical hollow bodies (HB) is less than the diameter of the quasi-horizontal section of the well (12). Preferably, the diameter of the cylindrical hollow body (HB) is not more than 50%, preferably not more than 40%, more preferably not more than 30% and especially preferably not more than 20% of the diameter of the quasi-horizontal section of the well (12). The length of the cylindrical hollow bodies (HB) depends on the length of the quasi-horizontal section of the well (12) which is to be blasted. The length may therefore, for example, be 10 to 2000 meters. It is also possible to use a plurality of cylindrical hollow bodies (HB) of shorter length. In this case, the length of the cylindrical hollow bodies (HB) is in the range from 10 to 20 meters. The cylindrical hollow bodies (HB) are not secured in the well (1), and instead float in the explosive (FE).

In process step (b), the hollow bodies (HB) and the detonatable free-flowing explosive (FE) may be introduced together or successively into the directional well. This means that it is possible to introduce first the hollow bodies (HB) and then the explosive (FE) into the directional well. In addition, it is possible to introduce first the explosive (FE) and then the hollow bodies (HB) into the directional well (1). It is also possible to introduce the hollow bodies (HB) together with the explosive (FE) into the directional well.

If first the hollow bodies (HB) and then the explosive (FE) are introduced into the directional well, cylindrical hollow bodies (HB), preferably pipes or tubes made from a polymer material, more preferably from polyethylene, are generally introduced into the directional well (1). Subsequently, the explosive (FE) is introduced into the directional well (1).

If spherical hollow bodies (HB) are used, they are preferably introduced into the directional well (1) together with the explosive (FE). In a preferred embodiment, in process step b), a detonatable free-flowing mixture (DM) comprising hollow bodies (HB), preferably spherical hollow bodies (HB), and the detonatable free-flowing explosive (FE), is used. The mixture (DM) comprises, as spherical hollow bodies (HB), preferably the above-described cenospheres, for which the details and preferences given apply correspondingly.

The present invention thus also relates to a process in which a detonatable free-flowing mixture (DM) is introduced in process step b) into the directional well (1) comprising hollow bodies (HB) and the explosive (FE).

In that case, the spherical hollow bodies (HB) are distributed, preferably dispersed, in the explosive (FE). The distribution or dispersion of the spherical hollow bodies (HB) is effected by methods known to those skilled in the art, for example by means of mixing units such as stirred tanks with propeller stirrers or dispersing stirrers.

The detonatable free-flowing explosives (FE) used may be slurries, emulsions or true solutions. Detonatable free-flowing explosives (FE) are known to those skilled in the art. These explosives are described, for example, in DE 3700129, RU 241678, RU 98101776 and RU 95113990.

The detonatable free-flowing explosives (FE) used may, for example, be mixtures of a fuel component, such as a hydrocarbon component, for example kerosene or mineral oil, and an oxidizing agent, for example ammonium nitrate or dinitrogen tetroxide (N₂O₄). These mixtures can also be used in the form of aqueous emulsions. The detonatable free-flowing explosive (FE) may additionally comprise customary explosives additives such as explosion sensitizers or explosion moderators.

A particularly preferred detonatable free-flowing explosive (FE) is a mixture of kerosene and liquid dinitrogen tetroxide (N₂O₄). A likewise preferred detonatable free-flowing explosive (FE) is an aqueous suspension of trotyl particles (TNT; IUPAC name 2-methyl-1,3,5-trinitrotoluene) comprising ammonium nitrate dissolved in water. The aqueous suspension is typically gelated by the addition of polyacrylamide or salts of carboxylated cellulose. The water content of the aqueous suspension may be up to 20% by weight. Such suspensions are available under the trade name “Aquatol®”.

“Free-flowing” in this context means that the explosive (FE) or the mixture (DM) can be introduced by pumping into the directional well (1), preferably into the quasi-horizontal section of the well (12).

The density (D_(FE)) of the detonatable free-flowing explosive (FE) is greater than the density (D_(HB)) of the hollow bodies (HB). The hollow bodies (HB) therefore have positive buoyancy in the explosive (FE). As a result of the density difference and the positive buoyancy, the hollow bodies (HB) float in the explosive (FE). The speed with which the hollow bodies (HB) float (the rising rate) depends essentially on three parameters. These are the density difference between the hollow bodies (HB) and the explosive (FE), the viscosity of the explosive (FE) and the adhesion of the explosive (FE) to the hollow bodies (HB).

The present invention thus also provides a process in which the hollow bodies (HB) have positive buoyancy in the detonatable free-flowing explosive (FE).

A greater density difference leads to an increase in the rising rate. An increase in the viscosity of the explosive (FE) leads to slowing of the rising rate. An increase in adhesion likewise leads to slowing of the rising rate.

It is advantageous in accordance with the invention that the hollow bodies (HB) float in the explosive (FE) with a certain time delay. This has the advantage that, in the embodiment in which a mixture (DM) is introduced into the directional well (1), the hollow bodies (HB) are distributed homogeneously in the explosive (FE). This facilitates the introduction of the mixture (DM) into the directional well (1). After a rest phase, the hollow bodies (HB) float in the mixture (DM).

A “rest phase” is understood in accordance with the invention to mean the time interval between the end of the introduction of the hollow bodies (HB) and of the explosive (FE), preferably in the form of the mixture (DM), and the initiation of the detonation.

The present invention thus also relates to a process in which the end of process step b) is followed by and process step c) is preceded by a rest phase in the range from 1 hour to 3 days.

The three parameters described above can be determined by laboratory tests. Suitable densities for the explosive (FE) are, for example, densities in the range from 0.95 g/cm³ to 2 g/cm³. The densities of the explosive (FE) are preferably in the range from 1 g/cm³ to 1.5 g/cm³. The density of the explosive (FE) can be increased by the addition of additives, for example glycerol, preferably crude glycerol, or salts such as sodium chloride or calcium chloride.

The viscosity of the liquid explosive (FE) is, for example, in the range from 50 to 1000 cP, preferably in the range from 200 to 600 cP. To increase the viscosity of the explosive (FE), thickening additives are added thereto. Suitable thickening additives are, for example, synthetic polymers such as polyvinyl alcohol, polyethylene oxide, polyvinylpyrrolidone, polyacrylamide, partly hydrolyzed polyacrylamide or natural polymers such as guar, glucans, xanthan, scleroglucan, hydroxypropyl cellulose, hydroxyethyl cellulose or hydroxymethyl cellulose.

The viscosities are determined in accordance with the invention by the method which follows. The viscosities were measured on a RheoStress 301, plate/cone arrangement, at shear rates of 0.5 to 1 s⁻¹.

The adhesion of the explosive (FE) to the hollow bodies (HB) can be regulated, for example, by the adjustment of the surface tension. Lowering of the surface tension leads to a reduction of adhesion. The surface tension can be lowered by adding surfactants, for example, to the explosive (FE). Suitable surfactants are anionic, cationic or nonionic surfactants. Suitable anionic surfactants are, for example, carboxylates or sulfonates having long alkyl radicals. Suitable cationic surfactants are, for example, quaternary ammonium compounds having long-chain alkyl radicals. Suitable nonionic surfactants are, for example, ethoxylates of fatty alcohols.

The hollow bodies (HB) and the explosive (FE) are preferably introduced into the quasi-horizontal section of the well (12). The mixture (DM) is preferably introduced into the quasi-horizontal section of the well (12). Particular preference is given to introducing the majority of the hollow bodies (HB) used and of the explosive (FE), preferably in the form of the mixture (DM), in the quasi-horizontal section of the well (12). A “majority” is understood to mean that at least 80% by weight, preferably at least 90% by weight, more preferably at least 95% by weight and especially preferably at least 99% by weight of the hollow bodies (HB) used and of the explosive (FE), preferably in the form of the mixture (DM), are introduced into the quasi-horizontal section of the well (12), based in each case on the total amount of the hollow bodies (HB) used and of the explosive (FE) or of the mixture (DM) used.

In a further, especially preferred embodiment, the hollow bodies (HB) and the explosive (FE) are introduced exclusively into the quasi-horizontal section (12), preferably in the form of the mixture (DM).

The present invention thus also relates to a process in which the majority of the hollow bodies (HB) used in process step b) and of the explosive (FE) are introduced into the quasi-horizontal part of the well (12).

This can be done, for example, by pumped introduction by means of a coiled tubing (30). For this purpose, a flexible pipe is introduced into the directional well (1). Preferably the mixture (DM) is introduced through the coiled tubing (30) into the quasi-horizontal section of the well (12). The mixture (DM) used with preference has a density (D_(DM)) greater than the density (D_(BF)) of the borehole fluid (BF). “Borehole fluid” is understood in accordance with the invention to mean the fluid present in the borehole before commencement of the process according to the invention. This may be formation water, mineral oil or mixtures/emulsions composed of formation water and mineral oil. The introduction of the mixture (DM) is followed by a rest phase. This rest phase may be one hour to three days. Through the rest phase, the hollow bodies (HB) float in the quasi-horizontal section of the well (12).

The state after flotation of the hollow bodies (HB) is shown schematically in FIG. 4. The flotation of the hollow bodies (HB) forms, in the upper cross-sectional region of the quasi-horizontal section of the well (12), a region having a lower density. Based on the cross section, principally the explosive (FE) is present in the lower region of the quasi-horizontal well (12). In the upper region, based on the cross section, are principally the hollow bodies (HB). This density difference enables guidance of the detonation. The detonation energy spreads predominantly in the direction of the hollow bodies (HB), which achieves vertical fissuring of the rock surrounding the quasi-horizontal section of the well (12). The state after the detonation is shown schematically in FIG. 6.

It is also possible to provide the mixture (DM) with a free-flowing tamping composition (7). The tamping composition serves to tamp the explosive (FE) in the quasi-horizontal section of the well (12). For this purpose, prior to the introduction of the explosive (FE) and of the hollow bodies (HB), preferably in the form of the mixture (DM), a preferably aqueous solution is first introduced into the borehole as a tamping composition (7) through a coiled tubing (30).

“Free-flowing” means that the tamping composition (7) can be introduced into the directional well (1) by pumping.

The present invention thus also relates to a process in which the introduction of the hollow bodies (HB) and of the explosive (FE) in process step b) is preceded by introduction of a tamping composition (7) into the directional well.

The aqueous solution used as the tamping composition (7) preferably likewise has a higher density and viscosity than the borehole fluid. This achieves effective displacement of the borehole fluid. The density of the tamping composition may be adjusted as described above for the adjustment of the density of the explosive (FE). It may also be advantageous to adjust the viscosity of the tamping composition (7) by the addition of thickeners. The thickeners used for this purpose may be the thickeners already listed above for the explosive (FE), for which the same details and preferences apply correspondingly.

The length of the tamping composition (7) in the directional well is generally in the range from 5 to 50 meters. Preference is also given to filling part of the quasi-horizontal section of the well (12), which is unlined or only temporarily lined with plastic pipes, with the tamping composition (7). This makes it possible to avoid damage to the section of the well (1) which has been lined with a feed pipe (8), i.e. permanently stabilized.

After the introduction of the tamping composition (7), the explosive (FE) and the hollow bodies (HB) are introduced via the coiled tubing (30) into the directional well (1), preferably in the form of the mixture (DM).

The density (D_(DM)) of the mixture (DM) and the viscosity are preferably higher than the density of the tamping composition (7). This achieves effective displacement of the tamping composition (7), which in turn displaces the borehole fluid.

The mixture (DM) may comprise 5 to 50% by volume of the hollow bodies (HB), based on the total volume of the mixture (DM) used in process step b).

The present invention thus also relates to a process in which the mixture (DM) comprises 5 to 50% by volume of hollow bodies (HB), based on the total volume of the mixture (DM) used in process step b).

Preference is given to filling only the quasi-horizontal section of the well (12) with hollow bodies (HB) and explosive (FE). Particular preference is given to filling only that part of the quasi-horizontal section of the well (12) which is not permanently stabilized.

The quasi-horizontal section of the well (12) may be filled completely with hollow bodies (HB) and explosive (FE). It is also possible to fill only parts of the quasi-horizontal section of the well (12). In addition, it is possible to serially fill parts of the quasi-horizontal section of the well (12), for example by introducing the tamping composition (7), followed by introduction of the mixture (DM), followed by another introduction of a tamping composition (7) and subsequent introduction of the mixture (DM). These steps can be repeated as often as desired. Through the introduction of tamping composition (7) and the introduction of the mixture (DM), it is possible to precisely establish the desired site at which the fracking is to take place in the quasi-horizontal section of the well (12).

In a further embodiment of the process according to the invention, in step b), a mixture (DM) comprising explosive (FE), hollow bodies (HB) and hollow bodies (HBn) is used. The hollow bodies (HBn) have a density (D_(HBn)) corresponding to the density (D_(FE)) of the explosive (FE). Thus, the hollow bodies (HBn) have neutral buoyancy in the explosive (FE) and do not float. The hollow bodies with neutral buoyancy (HBn) are thus distributed homogeneously in the mixture (DM) and do not float even after the rest phase.

The present invention thus also relates to a process in which a mixture (DM) additionally comprising hollow bodies (HBn) having a density (D_(HBn)), where (D_(HB)) corresponds to the density (D_(FE)) of the explosive (FE), is used in process step b).

The present invention further relates to a process in which a mixture (DM) additionally comprising hollow bodies (HBn) having neutral buoyancy in the explosive (FE) is used in process step b).

Through the proportion of the hollow bodies (HBn), it is possible to control the intensity of the detonation. In this embodiment, hollow bodies with positive buoyancy (HB) and hollow bodies with neutral buoyancy (HBn) are dispersed in the explosive (FE) and introduced together in step b) into the quasi-horizontal section of the well (12).

Process Step c)

In process step c), the detonation is initiated. “Detonation” is understood in accordance with the invention to mean the instantaneous conversion of the potential energy present in the explosive (FE) to form a shock wave, the shock wave attaining speeds between 1000 and 10 000 m/s, temperatures in the range from 2500 to 6000° C. and pressures in the range from 10 000 to 300 000 bar.

The explosive (FE) preferably comprises, as explained above, an oxidizing agent such as ammonium nitrate or dinitrogen tetroxide (N₂O) and a liquid fuel, for example mineral oil, gas condensate or kerosene. The explosive (FE) can additionally be used in the form of an aqueous emulsion. The liquid explosive (FE) is produced above ground by mixing the components with addition of the hollow bodies (HB). The mixture (DM) used with preference becomes detonatable only in the event of a local temperature rise to temperatures in the range from 600 to 1200° C. There is thus no hazard associated with handling above ground, in compliance with the legal requirements and the workplace safety regulations.

The detonation is typically initiated in process step c) by means of a detonator (24). The detonators used may be chemical or electrical detonators (24). Corresponding detonators are known and are described, for example, in EP 1 046 879.

The present invention thus also relates to a process in which the detonation in process step c) is initiated by a chemical or electrical detonator (24).

The detonators (24) used are preferably autonomous detonators. The autonomous detonators (24) preferably have a delay detonator which initiates the detonation in process step c) after a period of 1 to 3 days. The detonator (24) can be introduced into the directional well (1) in process step c) together with the hollow bodies (HB) and the explosive (FE), preferably in the form of the mixture (DM). It is also possible first to introduce the hollow bodies (HB) and the explosive (FE) into the directional part of the well and, in a subsequent step, to position the detonator (24) in the directional well (1), preferably in the quasi-horizontal section of the well (12).

If the detonator (24) used is a delay detonator, it is introduced into the directional well with the aid of the coiled tubing. The delay detonator (24) is mechanically secured at the end of the coiled tubing and inserted into the well. There is no need to insert the detonator (24) into the quasi-horizontal section of the well (12). The detonator is positioned in the free-flowing explosive (FE). It is also possible to introduce the detonator (24) into the well with a rope. Under the action of gravity, the detonator then reaches the region of the well filled with the free-flowing explosive (FE).

The chemical detonator used is preferably a combination of aqueous acid, preferably aqueous hydrochloric acid, and magnesium granules. To this end, for example, magnesium granules can be introduced into the directional well (1) in the form of an aqueous suspension and subsequently mixed with aqueous acid in the directional well (1). This forms a detonation mixture in the well (1), said mixture comprising magnesium granules and aqueous acid.

The aqueous acid used may, for example, be an aqueous hydrochloric acid solution having a hydrochloric acid content in the range from 1 to 38% by volume, preferably in the range from 10 to 25% by volume, more preferably in the range from 15 to 20% by volume.

The reaction of hydrochloric acid with magnesium gives hydrogen and heat, according to the following reaction equation:

2HCl+Mg=>MgCl₂+H₂+heat

The chemical reaction of one kilogram of magnesium with hydrochloric acid generates approx. 5000 kcal of heat, and the temperature of the detonation mixture reaches 600 to 800° C. This temperature initiates the detonation of the free-flowing explosive (FE).

If the quasi-horizontal section of the well (12) is filled serially, meaning that several sections of the borehole are filled with the mixture (DM), separated from one another by the tamping composition (7), several detonators (24) are positioned in the quasi-horizontal part of the well (12).

Since the density of the mixture (DM) is significantly reduced by the hollow bodies (HB) in the upper part of the cross section of the quasi-horizontal well (12), the detonation energy is directed predominantly vertically. The detonation forms a fissured zone (6) with numerous fissures (9) in the rock surrounding the quasi-horizontal well (12). A schematic diagram of the state after the detonation is shown in FIG. 6.

The length of the fissured zone (6) corresponds to the length of the quasi-horizontal section of the well (12) which was filled with the mixture (DM). In terms of cross section, the fissured zone (6) has an elliptical shape. The fissured zone (6) is above the quasi-horizontal section of the well (12) which has been destroyed. As a result, dense nonproductive strata (23) in particular are destroyed, these having separated the productive strata (2 a) of the deposit (2) from one another. This distinctly improves hydrodynamic communication of adjacent productive strata (2 a).

Through the direction/guidance of the detonation, principally rock formations and nonproductive intermediate strata (23) above the quasi-horizontal section of the well (12) are destroyed. The rock below the quasi-horizontal section of the well (12) is destroyed only to a minimal degree in the process according to the invention. This does not result in any damage to the rock strata which separate the productive strata (2 a) of the deposit (2) from the formation water below. As a result, waterlogging of the fissured zone (6) from the formation water-bearing strata is effectively prevented. After the explosion in process step c), a perforated pipe can be inserted for stabilization in the quasi-horizontal section of the well (12) from which the fissured zone (6) has formed.

After process step c), the production of raw materials from the underground formation can be continued by conventional processes.

The invention is illustrated by the working examples and figures which follow, but without being restricted thereto.

The individual figures show:

FIG. 1 vertical section of the directional well (1)

FIG. 2 vertical section of the directional well (1) prior to the detonation

FIG. 3 cross section of the quasi-horizontal section of the well (12) after introduction of the hollow bodies (HB) and the explosive (FE)

FIG. 4 cross section of the quasi-horizontal section of the well (12) after the rest phase, i.e. after flotation of the hollow bodies (HB)

FIG. 5 cross section of the quasi-horizontal section of the well (12) after the rest phase, following introduction of a mixture (DM) comprising explosive (FE), hollow bodies (HB) and hollow bodies (HB_(n)) with neutral buoyancy

FIG. 6 cross section of the fissured zone (6) after the explosion

FIG. 7 a cross section of the quasi-horizontal section of the well (12) into which a hermetic tubular hollow body (HB) as the hollow body (HB) and explosive (FE) have been introduced

FIG. 7 b cross section of the quasi-horizontal section of the well (12) into which a hermetic plastic pipe with a partial coating (20) as the hollow body (HB) and explosive (FE) have been introduced

FIG. 8 a cross section of a hermetic pipe section (10) with partial coating (21) on the inside

FIG. 8 b cross section of a hermetic pipe section (10) with partial coating (22) on the outside

FIG. 9 development scheme for an ore deposit

FIGS. 10 a, 10 b, 10 c phases of the introduction of the tamping composition (7) and the mixture (DM) into the quasi-horizontal section of the well (12)

FIG. 1 shows a vertical section through the directional well 1 with the quasi-vertical section of the well 11 and the quasi-horizontal section of the well 12 in the lower region of the deposit 2.

FIG. 2 shows a vertical section through the directional well 1. The quasi-vertical section of the well 11 and parts of the quasi-horizontal section of the well 12 are stabilized by a feed pipe 8. The quasi-horizontal section of the well 12 has been filled with the explosive (FE) 3 comprising hollow bodies (HB). The detonator 24 was introduced the explosive (FE) 3. The explosive (FE) 3 has been tamped with a tamping composition 7. Part of the quasi-horizontal section of the well 12 has likewise been filled with the tamping composition 7. This protects the part of the directional well 1 stabilized with the feed pipe 8 on detonation.

FIG. 3 shows a cross section of the quasi-horizontal section of the well 12 which has been filled with the explosive (FE) 3 comprising spherical hollow bodies 4. FIG. 3 shows the state directly after the filling of the quasi-horizontal section of the well 12 in the deposit 2. The hollow bodies (HB) 4 are distributed homogeneously in the explosive (FE) 3.

FIG. 4 shows the quasi-horizontal section of the well 12 after the rest phase. Owing to the positive buoyancy of the hollow bodies (HB) 4, they float in the explosive (FE) 3 and are present principally in the upper region of the cross section of the quasi-horizontal section of the well 12. This gives rise to a region with relatively low density in the upper region of the quasi-horizontal section of the well 12.

FIG. 5 shows a cross section of the quasi-horizontal section of the well 12, in which not only the spherical hollow bodies (HB) 4 with positive buoyancy but also spherical hollow bodies (HB) 5 with neutral buoyancy have been used.

FIG. 6 shows the cross section of the quasi-horizontal section of the well 12 after the detonation. The dotted circle with the reference numeral 13 describes the position of the cross section of the quasi-horizontal section of the well 12 prior to the explosion. FIG. 6 illustrates the fissured zone 6 which resulted from the detonation. The fissured zone 6 has an elliptical cross section with vertical elongation. The fissured zone 6 which resulted from the detonation is principally above the former quasi-horizontal section of the well 12 (illustrated in FIG. 6 by reference numeral 13). The detonation and the formation of the fissured zone 6 effectively destroyed the nonproductive intermediate strata 23 in the vertical direction, which distinctly improves hydrodynamic communication between the productive strata 2 a. At the edge of the fissured zone 6, there are additionally cracks and fissures 9 which improve hydrodynamic communication further.

FIG. 7 a shows an embodiment in which a tubular hollow body (HB) 10 and explosive (FE) 3 have been introduced into the quasi-horizontal section of the well 12.

FIG. 7 b shows an embodiment in which the hollow body (HB) introduced into the quasi-horizontal section of the well 12 has been a plastic pipe 10 having a partial coating 20. The plastic pipe 10 has not been secured in the quasi-horizontal section of the well 12, but floats in the explosive (FE) 3. The direction of buoyancy is symbolized by the black arrows.

FIGS. 8 a and 8 b show particular embodiments of the hollow body (HB) 10 with an internal or external partial coating of metal (steel or metal alloys).

FIG. 9 shows a development scheme for an ore deposit 25 for metal extraction. One injection well 26 and two production wells 27 have been sunk into the ore deposit 25. The quasi-horizontal sections of the injection well 26 and of the production wells 27 have been fracked by the process according to the invention. This has given rise to the fissured zones 61 and 62 in the ore deposit 25. An alkaline flooding solution 28 is injected through the injection well 26. The alkaline flooding solution 28 moves from the fissured zone 61 in the direction of the fissured zones 62 (symbolized in FIG. 9 by the unfilled arrows). As it passes through the ore deposit 25, the alkaline flooding solution 28 is enriched with the metal to be extracted in the form of soluble salts. This gives rise to the productive solution 29 enriched with the metal to be extracted. The productive solution 29 is forced by the pressure built up in the injection well 26 in the direction of the fissured zones 62 (symbolized in FIG. 9 by the black arrows). From the fissured zones 62, the productive solution is produced through the production well 27.

FIGS. 10 a, 10 b and 10 c show the phases for performance of process steps a) and b) according to the invention. In FIG. 10 a, the coiled tubing 30 was introduced into the quasi-horizontal section of the well 12. Through the coiled tubing 30, the tamping composition 7 is first injected into the quasi-horizontal section of the well 12, as a result of which the borehole fluid 71 is displaced. FIG. 10 b shows the introduction of the liquid explosive (FE) 3 and the hollow bodies (HB) into the quasi-horizontal section of the well 12. The explosive (FE) 3 and the hollow bodies (HB) are introduced in the form of the detonatable mixture (DM), likewise via the coiled tubing 30. This displaces the tamping composition 7, which in turn displaces the borehole fluid 71. FIG. 10 c shows the state after the introduction of the detonator 24 into the explosive (FE) 3 comprising the hollow bodies (HB) after removal of the coiled tubing 30. FIG. 10 c thus shows the state immediately prior to the initiation of the detonation in process step c). 

1. A process for fracking an underground formation, comprising at least the steps of a) sinking a directional well (1) comprising a quasi-vertical section (11) and a quasi-horizontal section (12) into the underground formation, b) introducing hollow bodies (HB) and a detonatable free-flowing explosive (FE) into the directional well (1) and c) initiating the detonation in the directional well (1), wherein the hollow bodies (HB) have a density (D_(HB)) and the detonatable free-flowing explosive (FE) has a density (D_(FE)) and (D_(HB)) is less than (D_(FE)), and the end of process step b) is followed by and process step c) is preceded by a rest phase, the result of which is that the hollow bodies (HB) float in the detonatable free-flowing explosive (FE).
 2. The process according to claim 1, wherein the hollow bodies (HB) have positive buoyancy in the detonatable free-flowing explosive (FE).
 3. The process according to claim 1, wherein the majority of the hollow bodies (HB) used in process step b) and of the explosive (FE) are introduced into the quasi-horizontal part of the well (12).
 4. The process according to claim 1, wherein the density (D_(HB)) of the hollow bodies (HB) is in the range from 0.2 to 0.9 g/cm³.
 5. The process according to claim 1, wherein the density (D_(FE)) of the explosive is in the range from 0.95 to 2 g/cm³.
 6. The process according to claim 1, wherein the hollow bodies (HB) used are spherical hollow bodies having a diameter in the range from 1 μm to 5 mm.
 7. The process according to claim 1, wherein the hollow bodies (HB) used are cenospheres having a shell comprising silicon oxide, aluminum oxide and iron oxide.
 8. The process according to claim 1, wherein the explosive (FE) comprises a fuel component and an oxidizing agent.
 9. The process according to claim 1, wherein a detonatable free-flowing mixture (DM) is introduced in process step b) into the directional well (1) comprising hollow bodies (HB) and the explosive (FE).
 10. The process according to claim 1, wherein the introduction of the hollow bodies (HB) and of the explosive (FE) in process step b) is preceded by introduction of a free-flowing tamping composition (7) into the directional well.
 11. The process according to claim 9, wherein the mixture (DM) comprises 5 to 50% by volume of hollow bodies (HB), based on the total volume of the mixture (DM) used in process step b).
 12. The process according to claim 9, wherein the rest phase is in the range from 1 hour to 3 days.
 13. The process according to claim 10, wherein a mixture (DM) additionally comprising hollow bodies (HBn) having neutral buoyancy in the explosive (FE) is used in process step b).
 14. The process according to claim 1, wherein the hollow bodies (HB) used are cylindrical hollow bodies having a partial metallic coating.
 15. The process according to claim 1, wherein the detonation in process step c) is initiated by a chemical or electrical detonator (24). 